A seismic streamer is an elongated cable-like structure (typically up to several thousand meters long), which includes an array of hydrophone cables and associated electric equipment along its length, and which is used in marine seismic survey. In order to perform a 3D/4D seismic survey, a plurality of such instrumented cables is towed behind a survey vessel. Acoustic signals are produced by the seismic sources are directed down through the water and into the seabed beneath, where they are reflected from various strata. The reflected signals are received by the hydrophone cables and then digitalized to build up a representation of the earth strata in the area being surveyed.
The instrumented cables are typically being towed at a constant depth of about five to ten meters to promote removal of undesired “ghost” reflections from the water surface. In order to keep the instrumented cables at a constant depth, control devices known as “birds” are arranged to each instrumented cable at intervals of 200 to 300 meters.
Low frequency depth variations and lateral movement is inevitable. The main reason for cable depth variations are long periodic waves. In general, the worst-case situation occurs when towing in the same direction as the swell. Cable lateral movements are mainly due to sea current components perpendicular to the towing direction. In the case of both swell and cross-current influences, the risk of instrumented cable entanglement is therefore increased.
The cable tension decreases proportionally with the distance from the towing point. Therefore, low frequency lateral and vertical movement tends to have higher amplitudes near closer to the tail. However, the forces acting perpendicular to the instrumented cables are non-uniformly distributed over the length of the cable and will change with time as the towed array moves forward.
During a seismic survey the instrumented cables are intended to remain straight, parallel to each other, equally spaced and at the same depth. However, after deploying the instrumented cables, it is typically necessary for the vessel to cruise in a straight line for at least three cable lengths before the cable distribution approximates this ideal arrangement and the survey can begin. This increases the time it takes to perform a survey, and therefore increases the cost of the survey. However, because of sea currents the instrumented cables fail to accurately follow the path of the seismic survey vessel, sometimes deviating from this path at an angle, known as feathering angle. This can adversely affect the coverage of the survey, frequently requiring that certain parts of the survey to be repeated. In really bad circumstances, the cables can actually become entangled, especially at the end of the instrumented cables, which can cause considerable damage and considerable financial loss.
Several patents cover control devices for instrumented cables, such as marine seismic streamers.
WO 02/059650 A1 describes a system which is characterized in that it includes a command unit and a number of control units, where the control units are located on at least some of the cables and are adapted to measure and report about their positions, that the command unit includes means for receiving position information from each individual control unit and calculation of possible deviations from predetermined positions, and that the system includes means for changing the cable position for re-establishing the positions of the control units.
WO 2005/096018 describes a method and a system which enables seismic survey spread while conducting a seismic survey. The method includes collecting input data, including navigation data for navigation nodes, operating states from sensors associated to spread control elements, environmental data for the survey and survey design data. The positions of the sources and the receivers are estimated using the navigation data, the operating states and the environmental data. Optimum towing tracks for the sources and receivers are determined using the position estimates and a portion of the input data that includes at least the survey design data. Drive commands are calculated for at least two of the spread control elements using the determined optimum towing tracks.
U.S. Pat. No. 6,691,038 B2 describes a method and apparatus including an active control system for a towed seismic cable array that enables relative position control of any number of towed seismic cables. The cable positions are controlled horizontally and vertically using active control units positioned within the seismic array. The three component (x, y, z) position for each cable element, relative to the vessel and relative to each other, is controlled, tracked and stored during a seismic data acquisition run. The active control elements can be located at diverter position, cable head, along the length of the cable or the cable tail. The method enables maneuvering of a seismic array as the towing vessel maintains course or the vessel itself maneuvers to assist the repositioning of the array.
U.S. Pat. No. 6,932,017; U.S. Pat. No. 7,162,967 B2; U.S. Pat. No. 7,222,579 B2 and U.S. Pat. No. 7,293,520 B2 do all describe methods and systems for controlling a cable positioning device arranged to be connected to an instrumented cable (marine seismic streamer) and towed by a seismic survey vessel and which have a wing and a wing motor to change the wing orientation. The method includes step for:—achieving an estimated velocity of the cable positioning device,—to calculate a desired change of wing orientation by using the estimated velocity of the cable positioning device and—to activate the wing motor to produce the desired change of the wing orientation.
From, among others, U.S. Pat. No. 5,443,027, U.S. Pat. No. 6,011,752, U.S. Pat. No. 6,144,342, WO 03/008906 A2, U.S. Pat. No. 6,671,223 B2 U.S. Pat. No. 6,879,542 B2, U.S. Pat. No. 6,459,653, U.S. Pat. No. 5,619,474, U.S. Pat. No. 6,016,286, U.S. Pat. No. 6,525,992 further control devices and methods for controlling instrumented cables are described.
U.S. Pat. No. 6,691,038 describes an active separation tracking and positioning system for towed seismic arrays.
U.S. Pat. No. 7,190,634 describes a GPS-based underwater cable positioning system.
U.S. Pat. No. 7,376,045 describe a system for determining position of towed marine seismic streamers.
U.S. Pat. No. 7,403,448 describe a streamer steering device orientation determination apparatus and methods.
Control devices according to these embodiments are attended with a number of disadvantages. As control devices (in some of the publications mentioned above) are suspended under the instrumented cable, they produce substantial noise when they are towed through the water. This noise is mixed with the reflected signals detected by the hydrophone cables in the instrumented cables. Some of the control devices include a pair of wings or rudders which are arranged to a rotatable structure encompassing the instrumented cable to generate a lifting force in a specified direction. This is a relatively expensive and complex electromechanical construction which is very vulnerable in subsea operations. Some of the control devices mentioned in the publications above are driven by a pair of wings or rudders in a “bank-to-turn” maneuver; such that the roll angle of the control device is defined from a desired total force. This adds complexity to the local control loop of the total control system and results in bad stability properties as roll angle must be adjusted continuously according to changes in desired total force, particularly induced from the depth control.
Moreover, solutions for performing wireless/contact-less transfer of energy and signals/data between main body and wings is not known, something which will result in a more robust system which is less exposed to functional failure or mechanical failure.
Some control devices which improve these disadvantages are known from the applicant's Norwegian patent 328856 and patent applications NO 20083830 and NO 20063182, and the applicant's application with the title “Control device and method for controlling the position of an instrumented cable towed in water”.